Sirzechs Lucifer, formerly known as Sirzechs Gremory, is the older brother of Rias Gremory. Known as the Crimson Satan, he is the leader of the Four Great Satans who rule the Underworld with the title of "Lucifer". He is also the husband of Grayfia Lucifuge and father of their son, Millicas Gremory. (Source: High School DxD Wikia)